To summarise, there are many things you can do, depending on whether there is functionality that you need:

Don't use hibernation? Then you can disable it and claim back a quantity of hard drive space equal to the amount of RAM you have, you just need to log on as an administrator, open a command prompt and type the following two lines:

powercfg –h offdel C:\hiberfil.sys

Old Windows Update files can be deleted as the folder they are stored in used to get quite big. They are stored in C:\Windows\SoftwareDistribution but you'll need to go through the method I detailed in the blog to properly clean the directory out.

System Restore is another hog and deleting old restore points can clean out space, you can even control how much space is available for use with System Restore

The WinSxS folder is a red herring and contains no real data that is not already duplicated elsewhere and deleting it will save you nothing. This special folder contains what is known as symbolic links to files which are scattered across your system and are kept in that folder to simplify matters slightly.

An alternative to the excellent CCleaner that Akira linked is a more commercially supported alternative from IOBits, Advanced SystemCare4. There is a free version available which will likely do just as much to clean out your system as CCleaner, but has a few added extras.

I've been playing with the WinSXS folder in a few test installs and I have figured out a way to save some space. In a 3 month old Windows 7 Pro install on my laptop, the SxS folder grew to 14GB. I can safely delete 7.4GB of that without any issues.

I was a bit surprised to read that most "technical" write ups say you can't delete anything in the SxS folder without issues. This is just not true and I wonder if anyone has tried different scenarios or if that opinion is just grown off of Microsoft's warnings. There is one thing that you can't do after "cleaning" the SxS folder, but it's rare that you'd need to so I won't even waste the keystrokes.

I've had this running smoothly for a LONG time, and on the laptop I use for work as an IT Director, so I put it through it's paces - that's for sure. It's well worth the disk space in some cases. For example -- I love my laptop which is an older Dell and I've got an SSD drive in it. The SSD is only 60 gig, so space is at a premium. As is, I use a 32GB SDCard in the onboard reader to give me extra space, but knocking a chunk of useless crap off is a nice option when you don't need 75% of the files in that folder.